'Super fog' leaves seven dead in massive US highway pileup

A large cloud of fog enveloping a highway led to at least 158 car crashes and seven deaths in the southern US state of Louisiana, authorities said Monday.

The so-called "super fog," which US media reported was caused by a mixture of marsh fires and dense fog, led to a massive pileup on Interstate 55, about 30 miles (48 kilometers) outside of New Orleans, the Louisiana State Police said in a statement.

Photos posted by the police showed both lanes of the highway bridge where the incident occurred blocked off by the wreckage, which included dozens of charred vehicles.

A fire had erupted on the highway, police said.

One vehicle appeared to have driven off the bridge and into the water below.

More than 25 people were taken to hospital, not counting the "many victims (who) sought medical assistance on their own."

Super fogs form "when a mixture of smoke and moisture released from damp smoldering organic material such as brush, leaves and trees, mixes with cooler, nearly saturated air," according to the National Weather Service.

When they occur, visibility can drop to less than 10 feet (three meters).